Voice over Internet Protocol (VoIP) generally describes the transmission of voice communication over IP networks. VoIP typically utilizes an Internet protocol (IP) phone. An IP phone may be hardware based or software based. Software-based IP phones are implemented on personal computers. Hardware-based IP phones are stand alone devices that resemble conventional telephones that operate on the plain old telephone system (POTS).
Some IP phones may operate on a network that includes an address translation device. Network Address Translation (NAT) modifies IP address and port information (e.g. transmission control protocol (TCP) or user datagram protocol (UDP)) in packet headers. A NAT device has IP addresses in a public address space. The devices on the internal network are assigned addresses in a private address space. The NAT device maps the private address space to an IP address in the public address space so that returning traffic can reach the internal devices. A specific type of NAT, Port Address Translation (PAT), involves the translation of both IP address and port numbers into a single IP address, that is the translation device's address. PAT devices are common in home or remote offices. IP phones on the internal network that rely on an address translation device may not communicate properly with IP phones on other networks.